Having built a base, or two, for racing seats, I'd say that 1/4
material is overkill. 1/8 material is sufficient and is what all of
the commercial set bases that I've seen are made from. Pretty easy to
bent 1/8 material by clamping it in a vise and whacking it with a hand
sledge. No heat is needed, nor is a lot of precision required.

I was at a race car fabrication shop today for an alignment, and the
guy their working on the race cars suggested using 1/4 steel as oppose
to 1/8. It's an overkill, but it's the only thing holding the seat in
when the car flips.